I would really use quad core processor, and the board would accommodate pci 16x, and if you are not running win 7 64 I would, put in some more fans, a 600 watt power supply, and a nice big CPU cooler...
The case should have lots of ventilation, and space for fans.
and then it depends on the motherboard "approved" list of video cards.
You will need to look that up, on the manufacturer website.
You are putting emphasis on the video card.
You should be starting with a better foundation, ie. a quad core processor.
You won't get the video you really want, until you do that first. The higher quality requires more free resources than you have now.
Then you can look up the video card, that is approved for your motherboard.